"Artificial" and "reconstituted" stone has long been made by man, many buildings dating from the Imperial Rome epoch being mainly constructed of a kind of concrete which was faced with marble. However, in this latter half of the twentieth century, concrete has proved to be less attractive as a building material because of the high atmospheric pollution obtaining in today's towns and cities, which conditions can destroy concrete, and even such natural materials as marble and sandstone, in the course of time.